Hydrogen is produced by steam methane reforming of a hydrocarbon containing feed in a steam methane reformer. A hydrocarbon containing stream, typically natural gas, is desulfurized and combined with superheated steam to provide a reactant feed stream. The resulting reactant feed stream is then heated within the convective section of the reformer and then introduced into catalyst packed reformer tubes located in a furnace section of the steam methane reformer. The catalyst is typically nickel. The furnace section has burners that provide heat to support endothermic steam methane reforming reactions in which methane reacts with the steam to produce a synthesis gas containing carbon monoxide and hydrogen. The flue gases produced by the combustion are then routed through a convective section of the steam methane reformer in which heat is recovered for preheating the reactant feed stream and for generating the superheated steam. The synthesis gas is then cooled and passes through one or more water-gas shift reactors in which the carbon monoxide reacts with steam to produce additional hydrogen and carbon dioxide. After further cooling, the hydrogen is separated from the synthesis gas stream by pressure swing adsorption and the resulting tail gas can be used in part to support combustion by the burners in the furnace section of the steam methane reformer.
Although natural gas is a common feed to a steam methane reformer, hydrocarbon streams containing hydrocarbons with more than two carbon atoms can also be processed. Common sources for such hydrocarbon streams include by-product streams of refineries, chemical production facilities and metal producing operations. In many cases these streams have an olefin content. For example, off-gas streams produced in refineries from processes such as fluidic catalytic cracking, coking, catalytic reforming, hydrocracking and etc. have a high hydrocarbon and generally, a moderate hydrogen content. The olefin content of such streams requires pretreatment to avoid carbon deposition on the reforming catalyst that would otherwise result in deactivation of such catalyst. Additionally, such streams have an organic sulfur content that is typically sufficiently high that the sulfur can also present a risk to the catalyst.
U.S. Pat. No. 7,037,485 discloses a steam methane reforming method in which a refinery off-gas alone or a mixture of the refinery off-gas and natural gas is preheated by heat exchange with the synthesis gas stream produced after a high temperature shift reactor and is then introduced into a reactor containing a sulfur tolerant catalyst that is capable of promoting both hydrogenation and oxidation reactions. Such a reactor can be operated in a hydrogenation mode in which hydrogen and the olefins within the feed stream contact the catalyst to hydrogenate the olefins into paraffins and to convert the organic sulfur content of the feed into hydrogen sulfide. The resulting product can then be introduced into a zinc oxide bed to adsorb the hydrogen sulfide and to produce a treated hydrocarbon containing stream that contains no more than about 0.1 ppm by volume of sulfur containing compounds and an olefin content of less than about 0.5 percent of olefins by volume on a dry basis. The reactor can alternatively be operated in an oxidative mode in which steam and oxygen are also introduced into the reactor to produce a stream having a similar reduced olefin and organic sulfur species content, but also, with an increased hydrogen content.
As indicated above, the catalyst used in the '485 patent is “sulfur tolerant”. However, this being said, if the refinery gas contains more than 20 ppmv dry organic sulfur species, when the reactor is switched to oxidation mode, the high temperatures and the presence of sulfur will result in the deactivation of the catalyst that will result in a reduction in catalytic activity upon a return to the hydrogenation mode. In this regard, the activity reduction in hydrogenation mode is proportional to the level of oxygen used in the oxidation mode when such sulfur species are present in amounts at and greater than the 20 ppmv level. It is to be noted that certain organic sulfur species that can be present in an off-gas, such as mercaptans, sulfides and thiophenes, are particularly difficult to remove upstream of the reactor. Therefore, such organic sulfur species will invariably be found in the off-gas to be processed.
For example, if a preheat temperature of a refinery off-gas is about 315° C. and the oxygen fed to the reactor is about 4 percent by volume of the refinery off-gas, the catalyst looses approximately 20 percent of activity when subsequently used in the hydrogenation mode when the feed to the reactor is at the 20 ppmv level. If the oxygen is about 6 percent of the refinery off-gas the catalyst looses approximately 50 percent of its activity. If the oxygen is about 8 percent of the refinery off-gas the catalyst is completely deactivated. The deactivation mechanism is not completely understood by the inventors herein, but the presence of sulfur, high temperatures and high hydrocarbon species present in the refinery gas probably combine to form coke that poisons the active catalyst sites. It has also been observed that the presence of organic sulfur in amounts that are greater than about 20 ppmv organic sulfur species in the oxidation mode will reduce reforming activity with less hydrogen produced and less destruction of the olefins.
As will be discussed, among other advantages of the present invention, the present invention provides a method and apparatus that uses such sulfur tolerant catalyst, as described above, utilized in a manner that deactivation of the catalyst can be virtually eliminated. Furthermore, such method and apparatus allows less of such catalyst to be used as compared to the prior art. Other advantages of the present invention will become apparent in the discussion that follows.